The present invention relates to multi-layered coatings to impart corrosion resistance to metal substrates and, in particular, to a combination of metal plating and organic coating layers for such substrates.
In areas where corrosion of ferrous metal substrates provide particular and pervasive problems, it is well known to utilize organic films such as paints and metallic coatings such as metal plating to minimize the effects of corrosion. Prior art in the general area of ferrous metal plating discloses nickel plating over an intermediate nickel zinc alloy plating (75 to 90% zinc), as in U.S. Pat. No. 1,564,581, and the use of zinc-rich, zinc-nickel alloy plating over a layer of copper or nickel plating, as in U.S. Pat. No. 2,419,231. Other uses of zinc-nickel plating layers are found in U.S. Pat. Nos. 4,282,073; 3,420,754; 4,407,900; and 4,314,893, and in Japanese Patent Publication No. 57-207199.
In automotive and other applications where relatively severe corrosive agents are found, and, in particular, for the metal fasteners used in such applications, improvements in corrosion resistance have been disclosed in U.S. Pat. Nos. 4,188,459 and 4,329,402, the disclosures of which are hereby incorporated by reference. Prior to the aforementioned patents, it was known that automotive fasteners can utilize sequential plating layers of copper, cadmium, copper, nickel and chromium or a chromium substitute such as tin-nickel, tin-cobalt or tin-cobalt-zinc alloys.
U.S. Pat. No. 4,188,459 discloses a multi-layered corrosion resistant plating for fasteners comprising a first micro-throwing alloy layer of nickel alloy followed by a layer of a galvanically protective metal or alloy such as cadmium, cadmium-tin, a dual layer of cadmium and tin, zinc or zinc alloy. Over this galvanically protective layer there is applied a layer of copper plating, followed by a layer of nickel plating, followed by a layer of chromium or metallic chromium substitute. U.S. Pat. No. 4,329,402 discloses the same first layer of a micro-throwing alloy, with the galvanically protective plating layer optionally applied next, and followed by an outer layer of chromate film or an organic coating such as paint.
While the aforementioned plating and coating layers provide good protection, it is advantageous to provide comparable or superior protection with a minimum of coating layers, for obvious cost reasons. While the galvanic protective layers of zinc are desirable, when they are utilized as the final plating layer there is often the problem of the production of an insoluble white corrosion product as they are sacrificially attacked by corrosive agents in use.
In the area of automotive fasteners where the fasteners are often applied manually on the assembly line there is additional problems of fatigue of the assembly worker due to the often high installation torques, and long drill times resulting from the use of high friction and thicker corrosion resistant coatings. Cadmium plating has provided lower friction to ferrous fasteners, but such plating has considerable drawbacks with respect to disposal of plating bath effluent containing cadmium metal and the cyanide often used in such baths, as well as the presence of poisonous metallic cadmium on the fastener.
Bearing in mind these and other deficiencies of the prior art, it is therefore an object of the present invention to provide superior corrosion resistant to ferrous metal substrates which are used in relatively severe corrosive environments such as those found in the automobile.
It is another object of the present invention to provide a corrosion resistant coating which is relatively low in cost yet is reliable in application and performance.
It is a further object of the present invention to provide a superior corrosion resisting protection for metal substrates having surface defects such as pits, cracks, laps, or voids.
It is another object of the present invention to provide the aforementioned corrosion resistant properties for ferrous metal fasteners, in particular.
It is a further object of the present invention to provide a corrosion resistant ferrous metal fastener which has a lower installation force or torque.
It is still another object of the present invention to provide improved fastener installation and corrosion resistance without the use of cadmium.